Impurity induced recrystallization in thoriated nickel

209

Materials Science and Engineering, 12 (1973) 209-215 'C, Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

Impurity Induced Recrystallization in Thoriated Nickel R. K. HOTZLER, S. BAHK, A. K. MISRA and L. S. CASTLEMAN Polytechnic Institute of Brooklyn, 333 Jay Street, Brooklyn, New York 11201, N.Y. (U.S.A.) (Received December 21, 1972)

Summary* We have established that thoriated nickel recrystallizes prematurely during annealing when its surface is exposed to impurity atoms of certain elements having low melting temperatures. The following elements produced this effect at one or more of the annealing temperatures employed in this investigation; sulfur, selenium, lead and bismuth. Through microstructural examination, we have concluded that an important mass transport mechanism underlying this phenomenon in thoriated nickel is liquid phase penetration, presumably along the original grain boundaries of the nickel. We have been able to rationalize our observations by formulating the following rule." if at the annealing temperature employed a liquid phase can form whose phase field is contiguous to the terminal nickel phase field, then premature recrystallization of the thoriated nickel will occur at that temperature. INTRODUCTION

It has been well known for many years that small amounts of nickel surface contaminant embrittle "doped" tungsten incandescent lamp wire by inducing recrystallization at low temperatures, which results in a brittle equi-axed structure 1. It was only recently, however, that a systematic investigation was undertaken to determine the kinetics of this recrystallization phenomenon and its underlying mechanisms 2"3. This work ztimulated us to investigate other dispersed phase systems, in order to see if impurity-induced recrystallization is unique to "doped" tungsten or if it is a more general phenomenon. We have concentrated our efforts on thoriated nickel and we have made a systematic * R6sum6 en fran~ais/~ la fin de l'article. Deutsche Z u s a m m e n f a s s u n g am SchluB des Artikels.

attempt to determine the effects of various impurity elements on the recrystallization behavior of this alloy. Two of us reported some preliminary results to the effect that thoriated nickel recrystallizes prematurely when exposed to an atmosphere containing small amounts of sulfur vapor 4. At about the same time, other investigators reported that cases of premature recrystallization appeared to have occurred when thoriated nickel sampl~ were exposed to brazing alloys containing silicon and beryllium 5. In this paper, we present a more detailed picture of the impurity-induced premature recrystallization phenomenon in thoriated nickel and, in particular, of the roles played by impurity elements such as sulfur, selenium, lead and bismuth. EXPERIMENTAL PROCEDURE

Materials Thoriated nickel was obtained from a commercial supplier in the form of 0.5 inch diameter rod in a stress-relieved condition. Its chemical composition (in volume percent with respect to ThO2) was as follows: Element

Weight percent.

Carbon Sulfur Copper Titanium Cobalt Iron Chromium (Thoria) Nickel

0.0015 0.0010 0.001 0.001 0.001 0.010 0.002 (2.3) Balance

The following elements were obtained from a commercial supplier in the form of rod, shot or

210 chips: tin, lead, antimony and bismuth. The purity was guaranteed by the supplier to be 99.999~o. Sulfur and selenium were obtained in the form of powder, 99.9~ pure. Procedure The specific sample design used depended upon whether the impurity element was solid or not at the annealing temperature used for the experiment. In the former case, diffusion couples were prepared in which one of the components was thoriated nickel and the other was a sample of the element which we wanted to diffuse into the dispersed phase alloy. In the latter case, a thoriated nickel sample was prepared in the form of a cylinder 0.5 inch in diameter and 0.5 inch long, with a section at one end reduced by machining to 0.375 inch in diameter and to a depth of about 0.200 inch. The planar surface having the reduced diameter was given a metallographic polish. A thoriated nickel cap was prepared of such dimensions that it could be press-fitted onto the reduced diameter of the specimen, leaving a small hermetically sealed space above the polished specimen surface. Then, a small amount of the impurity element was placed on the polished surface of the specimen, which was then covered by the press-fitted cap. For some experiments with lead, the latter was placed at the bottom of the cap, and the specimen was annealed in an inverted position, so that the liquid lead would remain out of contact with the polished surface. Prior to the annealing treatment, the specimen assembly was encapsulated in a 15 mm diameter vycor tube, the initial pressure being 10-5 to 10 -6 Torr. The capsules were annealed at various temperatures in an electrical resistance furnace controlled to within 2 deg C or less. Subsequently, the capsules were furnace cooled, the vycor tubes were broken, and longitudinal sections of the sample were prepared for microstructural examination. The electron photomicrographs were obtained from chromium shadowed carbon surface replicas prepared by means of standard acetate replicating tape techniques.

R.K. HOTZLERet al. and that premature recrystallization also occurred i n a sample annealed in contact with sulfur for 25 hours at 600°C (1112°F). The latter temperature is below the 637°C (1178°F) eutectic in the nickelsulfur alloy system. Since then, we have made additional runs for various times at temperatures of 6000C (ll12°F), 700°C (1292°F), 800°C (1472°F), 900°C (1652°F) and 1000°C (1832°F). We have found that premature recrystallization occurs in samples exposed to sulfur at temperatures of 700°C (1292°F) and above. However, by using replica electron microscopy, we have been unable to confirm the occurrence of premature recrystallization in samples exposed to sulfur at 600°C (ll12°F) for 25 hours. Figure 1 is an electron photomicrograph of asreceived thoriated nickel; typically, the average

Fig. 1. Electron micrograph of as-received thoriated nickel without any annealingtreatment.

RESULTS Exposure of thoriated nickel to sulfur Two of us reported earlier 4 that premature recrystallization occurred in a thoriated nickel sample exposed to sulfur for five hours at 1000°C (1832°F),

Fig. 2. Electron micrograph of thoriated nickel after exposure to a sulfur-bearingatmosphere for 3 hours at 800°C (1472°F).

IMPURITY INDUCED RECRYSTALLIZATIONIN THORIATED NICKEL

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is an electron photomicrograph of a thoriated nickel specimen after exposure to a sulfur-bearing atmosphere for 6 hours at 800°C (1472°F).

Fig. 3. Electron micrograph of thoriated nickel after exposure to a sulfur-bearingatmosphere for 25 hours at 800°'C(1472T), showing the presenceof a second phase along a grain boundary.

Fig. 4. Electron micrograph showing agglomerated thoria particles in a thoriated nickel specimen after exposure to a sulfur-bearing atmosphere for 6 hours at 800°C (1472°F). grain diameter is about 0.5/~m. In one sample we observed grain diameters in a small, highly localized region that were 4 or 5 times larger, but this was unusual. Figure 2 is an electron photomicrograph of thoriated nickel after exposure to a sulfurbearing atmosphere for 3 hours at 800°C (1472°F). The average grain diameter is very much larger and a second phase is present at the grain boundaries. Figure 3 is an electron photomicrograph ofthoriated nickel after exposure to a sulfur-bearing atmosphere for 25 hours at 800°C (1472°F). The second phase at the boundary between the two large grains is quite prominent. The particles within the grains are thoria particles that have agglomerated. A group of unusually large particles is shown in Fig. 4, which

Exposure of thoriated nickel to selenium As in the case with sulfur, two of us had reported that premature recrystallization occurred in a thoriated nickel sample exposed to selenium for 5 hours at 1000°C (1832°F) 4. We have run additional experiments involving exposure to selenium at temperatures of 700°C (1292 °F), 800 °C (1472 °F) and 900°C (1652°F). We have found that premature recrystallization occurs in the samples exposed to the impurity element at 800°C (1472°F) and at 900°C (1652°F), temperatures above that of the lower melting eutectic in the Ni-Se system, which is 750°C (1382°F). The recrystallization kinetics are apparently much slower in the selenium case than in the sulfur case, since we did not detect premature recrystallization in a sample annealed for 50 hours at 800°C (1472°F) and observed it only to a limited extent in a similar sample annealed for 120 hours. On the other hand, we observed no signs at all of premature recrystallization in a sample annealed for over 200 hours at 700°C (1292W). Exposure of thoriated nickel to lead Samples of thoriated nickel were annealed in contact with liquid lead for times ranging from 8 to 24 hours at temperatures of 800°C (1472°F), 850°C (1562°F), 930°C (1706°F) and 1000°C (1832 °F). A photomicrograph of a sample annealed for 8 hours at 1000°C (1832°F) is shown in Fig. 5. It is evident that premature recrystallization has occurred and that a continuous envelope of lead is present at

Fig. 5. Photomicrograph of thoriated nickel after exposure to liquid lead for 8 hours at 1000°C (1832°F) showing premature recrystallization. ( x 200)

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the grain boundaries. Similar results were found in the samples annealed at the other temperatures. Considerable erosion of the thoriated nickel sample surface occurred in contact with the liquid lead, and the boundary between the recrystallized portion of the sample and the unrecrystallized portion was highly diffuse and irregular. Consequently, it was not possible to study the recrystallization kinetics by using the movement inward of the recrystallization front as a parameter, as had been done in the work with tungsten 3. In an effort to improve the experimental conditions, we decided to reduce drastically the amount of lead in contact with the sample by resorting to experiments in which the samples were exposed to lead vapor. We varied the amount of lead vapor in the following way. In one group of specimens, we exposed the surface of a thoriated nickel specimen to lead vapor which was in equilibrium with a reservoir of liquid lead maintained at the same temperature as that of the thoriated nickel sample. In a second group of specimens, we saturated the liquid lead with nickel so as to reduce the pressure of the lead vapor in contact with the thoriated nickel sample. For example, at an annealing temperature of 10000C (1832°F) the sample was exposed to lead vapor at its pressure over pure liquid lead at that temperature. Experiments were performed also at 800°C (1472°F). The results were similar at both temperatures and are summarized below: (1) In the first type of experiment, we found that small humps had formed at discrete locations on the sample surface, which we identified as a leadrich phase. The thoriated nickel had recrystallized only in locations immediately underneath the humps and nowhere else. The lead-rich phase was present at the grain boundaries of the recrystallized grains and also occasionally as a spherical inclusion within a recrystallized grain. The thoria particles were coarsened considerably, as had occurred in samples exposed to sulfur. (2) In the second type of experiment, we observed neither humps on the surface nor recrystallization of the thoriated nickel.

Exposure of thoriated nickel to bismuth A sample of thoriated nickel was annealed in contact with liquid bismuth for 25 hours at 800°C (1472°F). The photomicrograph in Fig. 6 shows that premature recrystallization had occurred and that a second phase, presumably bismuth-rich, was present at the recrystallized grain boundaries.

R . K . HOTZLER et al.

Fig. 6. Electron micrograph of thoriated nickel after exposure to liquid bismuth for 25 hours at 800°C (1472°F) showing premature recrystallization.

Exposure of thoriated nickel to tin and antimony Samples of thoriated nickel were annealed in contact with liquid antimony for various times at temperatures of 800°C (1472°F) and 1000°C (1832°F), and in contact with liquid tin for various times at temperatures of 500°C (932°F), 600°C (ll12°F) and 1000°C (1832°F). There was no evidence of the occurrence of premature recrystallization in any of the samples. Exposure of thoriated nickel to other elements Samples of thoriated nickel were annealed in contact with samples of various elements for times and temperatures indicated in the table below: Element

Time (h)

Temperature C F

W Mo Cr V Fe Ti A1 Zn Be

1 2 2 2 2 2 20 20 20

1100 1100 1100 1100 1100 1100 600 400 1100

2012 2012 2012 2012 2012 2012 1112 752 2012

In no case was premature recrystallization observed. DISCUSSION

It is evident that impurity-induced premature recrystallization occurs not only in doped tungsten

IMPURITY I N D U C E D RECRYSTALLIZATION IN THORIATED NICKEL

but also in thoriated nickel ; thus, impurity-induced recrystallization may well be a general phenomenon in dispersed phase systems. In making this statement, however, we do not mean to imply that the mechanisms underlying the phenomenon in thoriated nickel are the same as those occurring in tungsten. On the contrary, we now believe that the mechanisms are different. In tungsten, the investigations of Brett et al. 2"3 show that solid state diffusion of nickel atoms from the surface into the interior plays an important role. In preliminary stages of our work in thoriated nickel with sulfur as the impurity element, two of us 4 speculated that the diffusion of sulfur atoms along high diffusivity paths would be one of the important mechanisms involved. We no longer believe this to be the case. Our observations are consistent with the concept that thoriated nickel exposed to a source of sulfur atoms recrystallizes prematurely because a liquid phase, presumably the low melting Ni-S eutectic, forms on the surface and penetrates into the interior. The evidence that has led us to this conclusion is as follows: (1) Small quantities of sulfur (less than 0.24~o) diffuse rapidly into thoriated nickel but do not cause premature recrystallization6. (2) In recrystallized regions of thoriated nickel samples, we always see a second phase enveloping the grain boundaries. (3) Premature recrystallization in thoriated nickel does not occur at 600°C (llI2°F), which is below the 637°C (1179°F) Ni-S eutectic; however, it does occur at 700°C (1292°F) and higher temperatures. (4) Others have reported that excessive grain growth in unalloyed nickel is caused by the penetration of a liquid sulfur-rich phase along the grain boundaries 7. Although mechanisms of recrystallization and grain growth are assuredly different, this observation may explain why we see very large grains present in the recrystallized portion of the thoriated nickel samples; the latter could have developed as a result of the presence of the liquid Ni-S eutectic. With the aid of a liquid phase penetration hypothesis, we can also explain our observations with respect to the behavior of lead as an impurity element. We rationalize the latter results as follows : When the surface of a thoriated nickel sample is exposed to vapor in equilibrium with unalloyed liquid lead, the rate at which the lead atoms arrive at and stick to the surface exceeds the rate at which they diffuse into the surface. Consequently, they nucleate small regions of lead-rich liquid alloy on the surface. The liquid penetrates the thoriated

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nickel along the grain boundaries and causes the observed recrystallization. However, if the activity of the lead vapor is reduced to that over a liquid alloy of lead saturated with nickel, the thermodynamic driving force for the formation of the liquid agglomerations is eliminated and instead the thoriated nickel surface remains solid and becomes saturated with lead. Since the solubility of lead is quite limited in nickel (about 1.2 at.~o at 1340°C (2444°F)) 8, the sample surface should saturate early during the annealing process. The fact that premature recrystallization is not observed in the latter type of experiment must indicate that the solid state diffusion of lead atoms in the numbers available is insufficient to activate the recrystallization process. We may gain further insight by comparing the effects upon thoriated nickel of exposure to low melting impurity elements; the results of our work are listed in Table 1. From these observations, we draw the following conclusions: (1)Premature recrystallization occurs in thoriated nickel whenever a terminal liquid impurity-rich phase is contiguous to the nickel-rich terminal phase; this condition is satisfied in the Ni-Pb and Ni-Bi phase diagrams (and the Ni-Se phase diagrams at 1000°C (1832°F)) for the annealing temperatures used in our studies. (2) Premature recrystallization does not occur in thoriated nickel whenever a solid intermediate alloy phase separates the terminal impurity-rich liquid phase from the terminal nickel-rich phase. This condition is satisfied in the Ni-Sb and Ni-Sn phase diagrams; it is also satisfied in the Ni-S and Ni-Se phase diagrams for annealing temperatures below their respective eutectic temperatures. Unfortunately, there is at present insufficient information to determine whether the Ni-Te phase diagram meets this condition. (3) Premature recrystallization occurs in thoriated nickel whenever an intermediate liquid phase (arising on heating through a eutectic reaction) is contiguous to the nickel-rich phase. This condition is satisfied by the Ni-S and N~Se phase diagrams for annealing temperatures above their respective eutectic temperatures. In comparing impurity-induced premature recrystallization in "doped" tungsten with that in thoriated nickel, we have come to the conclusion that the mass transport mechanisms involved are different. As we mentioned earlier, in the former case, solid state diffusion is the mechanism whereby the impurity moves from the surface into the interior ;

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R. K. H O T Z L E R et al.

TABLE 1 Effects of low-melting impurity elements

Impurity element

Premature recrystallization

Annealin9 temperature (°C)

Meltin9 temperature (°C)

Eutectic formation (°C)

Phase adjacent to nickel-rich terminal phase

S S Pb Bi Se Se Se Te Sb Sn

No Yes Yes Yes Yes 4 Yes No No 4 No No

600 700-1000 800-1000 800 1000 800- 900 700 1000 800-1000 50(01000

119 119 327 271 217 217 217 449 631 232

637 637 None None 750 750 750 ? 1070 1130

NiaS 2 (solid) b S-rich liquid b Pb-rich liquid b Bi-rich liquid" Se-rich liquid b Se-rich liquid b Ni3Se 2 (solid) b 40 a t . ~ Te fl (solid)? c NisSb 2 (solid) a Ni3Sn (solid) a

" M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958: Bi-Ni, p. 322; Ni-Sb, p. 1037; Ni-Sn, p. 1043. b R. P. Elliott, Constitution of Binary Alloys, First Supplement, McGraw-Hill, New York, 1965; N i - P b , p. 663; Ni-S, p. 669; Ni-Se, p. 673; Ni-Te, p. 675. c F. A. Shunk, Constitution of Binary Alloys, Second Supplement, McGraw-Hill, New York, 1969; Ni-Te, p. 558.

in the latter case, we feel we have demonstrated that the impurity is transported into the interior through liquid phase penetration. At this stage of the investigation, we have no information on any of the details of the liquid phase penetration process. However, judging from the work of Cheney et al. 9 and Bishop e t al. 1°'11 on the penetration of liquid bismuth along the grain boundaries of unalloyed nickel, we expect the process to be quite complicated.

CONCLUSIONS

In this investigation, we have established that thoriated nickel recrystallizes prematurely during annealing when its surface is exposed to impurity atoms of certain elements having low melting temperatures. The following elements produced this effect at the annealing temperatures indicated : sulfur, 700-1000°C (1292-1832°F); selenium, 8001000°C (1472-1832°F); lead, 800-1000°C (14721832°F); and bismuth, 800°C (1472°F). We found that the following low-melting temperature elements did not induce premature recrystallization at the temperatures indicated: sulfur, 600°C (ll12°F); selenium, 700°C (1292°F); antimony, 800-1000°C (1472-1832°F); and tin, 500-1000°C (932-1832°F). Also, we detected no evidence that premature recrystallization occurred when thoriated nickel was annealed in contact with the following elements at l l00°C (2012°F): tungsten, molybdenum, chromium, vanadium, iron, titanium and beryllium. This

observation was valid likewise for aluminum and zinc for annealing temperatures of 600°C (1112°F) and 400°C (752°F), respectively. We have concluded, on the basis of microstructural examination for the most part, that an important mechanism underlying this phenomenon in thoriated nickel is mass transport of the impurity by liquid phase penetration, presumably along the original grain boundaries. Therefore, we believe that impurity-induced premature recrystallization in thoriated nickel is basically different from nickelinduced premature recrystallization in "doped" tungsten 2'3 ; in the latter case, solid state diffusion o'f nickel atoms into the tungsten is the mass transport mechanism involved. Using the liquid phase penetration concept, we have been able to rationalize our observations by formulating the following rule: if at the annealing temperature employed a liquid phase can form that is contiguous to the nickel terminal phase in the binary phase diagram, then recrystallization of the thoriated nickel will occur at that temperature.

ACKNOWLEDGEMENTS This work was supported by the National Science Foundation under Grant No. GK 14708. REFERENCES l R. Nelson, Sylvania Technologist, 1957, Vol. 10, pp. 78-83.

IMPURITY INDUCED RECRYSTALLIZATION IN THORIATED NICKEL 2 T. Montelbano, J. Brett, L. Castleman and L. Seigle, Trans. AIME, 242 (1968) 1973. 3 S. Friedman and J. Brett, Trans. AIME, 242 (1968) 2121. 4 R. K. Hotzler and L. S. Castleman, Trans. AIME, 242 (1968) 750. 5 L. Godfrey, H. A. Hauser, S. G. Berkley and E. F. Bradley, in G. S. Ansell, T. D. Cooper and F. V. Lenel (eds.), Oxide Dispersion Strengthenin9, Gordon and Breach, New York, 1968, pp. 885-905.

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6 R. K. Hotzler and L. S. Castleman, Met. Trans., 3 (1972) 2561. 7 J.T. Smith and C. W. Spencer, Trans. AIME, 227 (1963) 783. 8 R. P. Elliott, Constitution of Binary Alloys, First Supplement, McGraw-Hill, New York, 1965, pp. 662-664. 9 R. F. Cheney, F. G. Hochgraf and C. W. Spencer, Trans. AIME, 221 (1961) 492. 10 G. H. Bishop, B. F. Addis, C. A. Steidel and C W. Spencer, Trans. AIME, 224 (1962) 1299. 11 G. H. Bishop, Trans. AIME, 242 (1968) t343.

Recristallisation induite par les impuretOs dans le nickel-thorine

Durch Thorium-Verunreinigungen kristallisation in Nickel

induzierte Re-

Les auteurs ont 6tabli que le nickel-thorine recristallise pr6matur6ment lors du recuit, lorsque sa surface est expos6e ~ des atomes d'impuret6s de certains 616ments ~ bas point de fusion. Les 616ments qui produisent cet effet, ~t l'une ou l'autre des temp6ratures de recuit utilis6es dans cette 6tude, sont les suivants: soufre, s616nium, plomb et bismuth. Les examens de microstructure montrent que dans le nickel-thorine le m6canisme de transport en masse, responsable du ph6nom6ne observ6, est la p6n6tration en phase liquide, probablement le long des joints de grains initiaux du nickel. Les auteurs ont pu expliquer leurs observations en formulant la r6gle suivante: le nickelthorine recristallise de fa~on pr6matur6e ~ une certaine temp6rature, lorsqu'fi cette temp6rature une phase liquide peut se former et que cette phase se situe dans le domaine contigu ~tcelui de la solution solide terminale riche en nickel.

Wir konnten zeigen, dab thorium-verunreinigtes Nickel beim Anlassen frfihzeitig rekristallisiert wenn seine Oberfl~iche Verunreinigungsatomen bestimmter Elemente ausgesetzt ist. Folgende Elemente ffihrten bei einer oder bei mehreren der angewandten AnlaBtemperaturen zu diesem Effekt: Schwefel, Selen, Blei und Wismut. Gefiigeuntersuchungen haben gezeigt, dab das Eindringen der fliissigen Phase in das thorium-verunreinigte Nickel (vermutlich entlang der ursprfinglichen Korngrenzen des Nickel) als einer der grundlegenden Mechanismen des Massetransports ffir dieses Ph~inomen anzusehen ist. Wir konnten unsere Beobachtungen durch Formulierung der folgenden Regel erkl~iren: Wenn sich bei der Anlal3temperatur eine flfissige Phase bilden kann, deren Phasenbereich auf der Nickelseite zusammenh~ingend ist, so finder bei dieser Temperatur friihzeitige Rekristallisation des thorium-verunreinigten Nickel statt.